1. Field of the Invention
The invention relates to a fuel injection system for directly injecting fuel into a cylinder of an internal combustion engine and, more particularly, to a fuel injection system for performing fuel injections a plurality of times per cycle of an internal combustion engine.
2. Related Art
There is known a fuel injection system which accumulates high pressure fuel supplied from a fuel pump in a common rail, and supplies the accumulated high pressure fuel to a fuel injector which responsively open at a predetermined fuel injection timing, thereby directly injecting the fuel into a cylinder of an internal combustion engine.
This fuel injection system is used not only for a diesel engine for compressing and igniting fuel mixture but also a gasoline engine for igniting the fuel mixture by a spark generated by a spark plug. In recent years, combustion efficiency is improved by executing an injection of fuel from the fuel injector by a pre-injection and a main injection per cycle of the internal combustion engine, as proposed in JP-A 4-252830 and SAE 980505, for instance.
In such conventional fuel injection systems, however, since it is set so that the half or more of the entire fuel to be injected into the cylinder of the internal combustion engine is injected by the pre-injection and the fuel is distributed into the whole area in the cylinder, the mixture in the whole area in the cylinder becomes lean by the pre-injection. The fuel mixture is partially pre-ignited by compression in the cylinder by movement of a piston, thereby tending to cause knocking. When the occurrence of the knocking is tried to be suppressed, preferable engine performance cannot be obtained.
In another fuel supply system according to U.S. Pat. No. 5,694,903 for supplying a fuel from a high pressure fuel supplying pump to a fuel injector, a needle lift is changed by a fuel pressure to vary a characteristic of the rate of injection. In this fuel injection system, however, it is necessary to change a driving power supplied to a solenoid in two phases, thus resulting in a complicated and expensive driving system.
Although the rate of injection can be variably controlled, variable control of spray conditions such as the angle of spray and spray penetration cannot be realized. When the spray conditions are not properly controlled, the fuel consumption worsens and production of NOx, soot, HC, and the like increases.
Further, JP-U-5-24956 discloses a swirl type fuel injector for promoting atomization of fuel and controlling spray characteristics, for example, a fuel injector in which a member for swirling a fuel is provided in the nozzle body, JP-A 3-175148 discloses a fuel injector in which a needle is provided with a fuel swirling mechanism, and U.S. Pat. No. 5,170,945 discloses a fuel injector in which a spray pattern of a fuel is changed by a lift of the needle. It is, however, difficult to obtain a preferable spray pattern.